Calcium phosphate compounds, in particular hydroxyapatite, have an excellent biocompatibility and their use as biomaterials in medical or dental fields has been widely investigated.
Calcium phosphate compounds other than apatite are known to be converted to hydroxyapatite upon hydrolysis under certain conditions (such as acidic conditions) and set at the same time. In an attempt to exploit this phenomenon, several efforts have been made to use calcium phosphate powders as dental or medical cement materials (as described, for example, JP-A-63-12705, 62-161206, 59-182263 and 59-88351) (the term "JP-A" as use an unexamined published Japanese patent application).
Calcium phosphate powders are set by mixing them with a setting solution. In order to ensure that the setting material will do no harm to body tissues, it is desirable for the mixture to have a low pH and that the setting reaction will proceed in the neutral range. An aqueous solution of citric acid has heretofore been considered the best setting solution capable of satisfying these requirements. However, such an aqueous solution of citric acid still leaves room for improvement, especially when compared with acrylic cement materials which have been commonly used in the art. The considerable problem is that a mixture of a calcium phosphate powder and an aqueous citric acid solution is not gum-like and cannot be readily formed into a desired shape.